The first catalytic asymmetric (4+3) cyclization of in situ generated ortho-quinone methides with 2-indolylmethanols has been established, which constructed seven-membered heterocycles in high yields (up to 95 %) and excellent enantioselectivity (up to 98 %). This approach not only represents the first catalytic asymmetric (4+3) cyclization of o-hydroxybenzyl alcohols, but also enabled an unprecedented catalytic asymmetric (4+3) cyclization of 2-indolylmethanols. In addition, a scarcely reported catalytic asymmetric (4+3) cyclization of para-quinone methide derivatives was accomplished.

Catalytic Asymmetric (4+3) Cyclizations of In Situ Generated ortho-Quinone Methides with 2-Indolylmethanols.

The merging of click chemistry with discrete photochemical processes has led to the creation of a new class of click reactions, collectively known as photoclick chemistry. These light-triggered click reactions allow the synthesis of diverse organic structures in a rapid and precise manner under mild conditions. Because light offers unparalleled spatiotemporal control over the generation of the reactive intermediates, photoclick chemistry has become an indispensable tool for a wide range of spatially addressable applications including surface functionalization, polymer conjugation and cross-linking, and biomolecular labeling in the native cellular environment. Over the past decade, a growing number of photoclick reactions have been developed, especially those based on the 1,3-dipolar cycloadditions and Diels-Alder reactions owing to their excellent reaction kinetics, selectivity, and biocompatibility. This review summarizes the recent advances in the development of photoclick reactions and their applications in chemical biology and materials science. A particular emphasis is placed on the historical contexts and mechanistic insights into each of the selected reactions. The in-depth discussion presented here should stimulate further development of the field, including the design of new photoactivation modalities, the continuous expansion of λ-orthogonal tandem photoclick chemistry, and the innovative use of these unique tools in bioconjugation and nanomaterial synthesis.

Light-Triggered Click Chemistry.

The combination of light activation and N-heterocyclic carbene (NHC) organocatalysis has enabled the use of acid fluorides as substrates in a UVA-light-mediated photochemical transformation previously observed only with aromatic aldehydes and ketones. Stoichiometric studies and TD-DFT calculations support a mechanism involving the photoactivation of an ortho-toluoyl azolium intermediate, which exhibits "ketone-like" photochemical reactivity under UVA irradiation. Using this photo-NHC catalysis approach, a novel photoenolization/Diels-Alder (PEDA) process was developed that leads to diverse isochroman-1-one derivatives.

https://onlinelibrary.wiley.com/doi/epdf/10.1002/anie.201914456

N-Heterocyclic Carbene Catalyzed Photoenolization/Diels-Alder Reaction of Acid Fluorides.

Fluorinated alcohols have been widely used in the synthetic organic chemistry over the past decades. The unique properties such as the strong hydrogen-bonding donor ability and low nucleophilicity allow them to promote organic reactions in the absence of any catalyst. These approaches have distinct advantages in terms of operational simplicity, practicability and environmental friendliness. Reactions promoted by fluorinated alcohols, including nucleophilic substitution reactions, annulation reactions, electrophilic reactions, dearomatization reactions, functionalization of multiple bond, epoxidation reactions and miscellaneous reactions have been summarized in this account.

Fluorinated Alcohols: Magic Reaction Medium and Promoters for Organic Synthesis.

We describe herein a highly diastereo- and enantioselective [4+3]-cycloannulation of ortho-quinone methides and carbonyl ylides to furnish functionalized oxa-bridged dibenzooxacines with excellent yields and stereoselectivity in a single synthetic step. The combination of rhodium and chiral phosphoric acid catalysis working in concert to generate both transient intermediates in situ provides direct access to complex bicyclic products with two quaternary and one tertiary stereogenic centers. The products may be further functionalized into valuable and enantiomerically highly enriched building blocks.

https://onlinelibrary.wiley.com/doi/epdf/10.1002/anie.201913603

Cooperative Catalysis for the Highly Diastereo‐ and Enantioselective [4+3]‐Cycloannulation of ortho‐Quinone Methides and Carbonyl Ylides

In recent decades, transient and highly reactive ortho-quinodimethanes (o-QDMs), ortho-quinone methides (o-QMs) and aza-ortho-quinone methides (aza-o-QMs) have attracted much attention and have been extensively studied and applied in organic synthesis, especially natural product total synthesis. This review summarizes recent advances in Diels–Alder reactions involving in situ-generated o-QDMs, o-QMs and aza-o-QMs, highlighting the power and potential of this strategy in organic synthesis and natural product total synthesis. An overview of the methods for generating these intermediates is also available.

Recent advances in the application of Diels-Alder reactions involving o-quinodimethanes, aza-o-quinone methides and o-quinone methides in natural product total synthesis.

Shuanhu Gao

Papers

Recent advances in the application of Diels-Alder reactions involving o-quinodimethanes, aza-o-quinone methides and o-quinone methides in natural product total synthesis.